Publications


Classification of macroscopic quantum effects

Optics Communications (2014)

T Farrow, V Vedral

We review canonical experiments on systems that have pushed the boundary between the quantum and classical worlds towards much larger scales, and discuss their unique features that enable quantum coherence to survive. Because the types of systems differ so widely, we use a case by case approach to identifying the different parameters and criteria that capture their behaviour in a quantum mechanical framework. We find it helpful to categorise systems into three broad classes defined by mass, spatio-temporal coherence, and number of particles. The classes are not mutually exclusive and in fact the properties of some systems fit into several classes. We discuss experiments by turn, starting with interference of massive objects like macromolecules and micro-mechanical resonators, followed by self-interference of single particles in complex molecules, before examining the striking advances made with superconducting qubits. Finally, we propose a theoretical basis for quantifying the macroscopic features of a system to lay the ground for a more systematic comparison of the quantum properties in disparate systems. © 2014 Elsevier B.V. All rights reserved.


Multipartite entangled spatial modes of ultracold atoms generated and controlled by quantum measurement

Physical Review Letters American Physical Society 114 (2014) 113604

T Elliott, W Kozlowski, S Caballero-Benitez, I Mekhov

We show that the effect of measurement back-action results in the generation of multiple many-body spatial modes of ultracold atoms trapped in an optical lattice, when scattered light is detected. The multipartite mode entanglement properties and their nontrivial spatial overlap can be varied by tuning the optical geometry in a single setup. This can be used to engineer quantum states and dynamics of matter fields. We provide examples of multimode generalizations of parametric down-conversion, Dicke, and other states, investigate the entanglement properties of such states, and show how they can be transformed into a class of generalized squeezed states. Further, we propose how these modes can be used to detect and measure entanglement in quantum gases.


Scale-estimation of quantum coherent energy transport in multiple-minima systems.

Scientific reports Nature Publishing Group 4 (2014) 5520-

T Farrow, V Vedral

A generic and intuitive model for coherent energy transport in multiple minima systems coupled to a quantum mechanical bath is shown. Using a simple spin-boson system, we illustrate how a generic donor-acceptor system can be brought into resonance using a narrow band of vibrational modes, such that the transfer efficiency of an electron-hole pair (exciton) is made arbitrarily high. Coherent transport phenomena in nature are of renewed interest since the discovery that a photon captured by the light-harvesting complex (LHC) in photosynthetic organisms can be conveyed to a chemical reaction centre with near-perfect efficiency. Classical explanations of the transfer use stochastic diffusion to model the hopping motion of a photo-excited exciton. This accounts inadequately for the speed and efficiency of the energy transfer measured in a series of recent landmark experiments. Taking a quantum mechanical perspective can help capture the salient features of the efficient part of that transfer. To show the versatility of the model, we extend it to a multiple minima system comprising seven-sites, reminiscent of the widely studied Fenna-Matthews-Olson (FMO) light-harvesting complex. We show that an idealised transport model for multiple minima coupled to a narrow-band phonon can transport energy with arbitrarily high efficiency.


Universal optimal quantum correlator

International Journal of Quantum Information 12 (2014)

F Buscemi, M Dall'Arno, M Ozawa, V Vedral

© 2014 World Scientific Publishing Company. Recently, a novel operational strategy to access quantum correlation functions of the form Tr[AρB] was provided in [F. Buscemi, M. Dall'Arno, M. Ozawa and V. Vedral, arXiv:1312.4240]. Here we propose a realization scheme, that we call partial expectation values, implementing such strategy in terms of a unitary interaction with an ancillary system followed by the measurement of an observable on the ancilla. Our scheme is universal, being independent of ρ, A, and B, and it is optimal in a statistical sense. Our scheme is suitable for implementation with present quantum optical technology, and provides a new way to test uncertainty relations.


Work and quantum phase transitions: quantum latency.

Physical review. E, Statistical, nonlinear, and soft matter physics 89 (2014) 062103-

E Mascarenhas, H Bragança, R Dorner, M França Santos, V Vedral, K Modi, J Goold

We study the physics of quantum phase transitions from the perspective of nonequilibrium thermodynamics. For first-order quantum phase transitions, we find that the average work done per quench in crossing the critical point is discontinuous. This leads us to introduce the quantum latent work in analogy with the classical latent heat of first order classical phase transitions. For second order quantum phase transitions the irreversible work is closely related to the fidelity susceptibility for weak sudden quenches of the system Hamiltonian. We demonstrate our ideas with numerical simulations of first, second, and infinite order phase transitions in various spin chain models.


Transitionless quantum driving in open quantum systems

NEW JOURNAL OF PHYSICS 16 (2014) ARTN 053017

G Vacanti, R Fazio, S Montangero, GM Palma, M Paternostro, V Vedral


Discord as a Quantum Resource for Bi-Partite Communication

ELEVENTH INTERNATIONAL CONFERENCE ON QUANTUM COMMUNICATION, MEASUREMENT AND COMPUTATION (QCMC) 1633 (2014) 116-118

HM Chrzanowski, M Gu, SM Assad, T Symul, K Modi, TC Ralph, V Vedral, PK Lam


Work and quantum phase transitions: Quantum latency

PHYSICAL REVIEW E 89 (2014) ARTN 062103

E Mascarenhas, H Braganca, R Dorner, M Franca Santos, V Vedral, K Modi, J Goold


Quantum zeno dynamics induced by light scattering from ultracold atoms in optical lattices

Optics InfoBase Conference Papers (2014)

G Mazzucchi, SF Caballero Benitez, IB Mekhov


Quantum entanglement

NATURE PHYSICS 10 (2014) 256-258

V Vedral


Experimental verification of quantum discord in continuous-variable states and operational significance of discord consumption

Conference on Lasers and Electro-Optics Europe - Technical Digest 2014-January (2014)

S Hosseini, S Rahimi-Keshari, JY Haw, SM Assad, HM Chrzanowski, J Janousek, T Symul, TC Ralph, PK Lam, M Gu, K Modi, V Vedral

© 2014 Optical Society of America. We introduce a simple and efficient technique to verify quantum discord in unknown Gaussian states and certain class of non-Gaussian states. We show that any separation in the peaks of the marginal distributions of one subsystem conditioned on two different outcomes of homodyne measurements performed on the other subsystem indicates correlation between the corresponding quadratures and hence nonzero quantum discord. We also demonstrate that under certain measurement constraints, discord between bipartite systems can be consumed to encode information that can only be accessed by coherent quantum interaction.


Experimental verification of quantum discord in continuous-variable states and operational significance of discord consumption

2014 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (2014)

S Hosseini, S Rahimi-Keshari, JY Haw, SM Assad, HM Chrzanowski, J Janousek, T Symul, TC Ralph, PK Lam, M Gu, K Modi, V Vedral, IEEE


The uncertainty principle enables non-classical dynamics in an interferometer

Nature communications 5 (2014) 4592

OCO Dahlsten, AJP Garner, V Vedral

The quantum uncertainty principle stipulates that when one observable is predictable there must be some other observables that are unpredictable. The principle is viewed as holding the key to many quantum phenomena and understanding it deeper is of great interest in the study of the foundations of quantum theory. Here we show that apart from being restrictive, the principle also plays a positive role as the enabler of non-classical dynamics in an interferometer. First we note that instantaneous action at a distance should not be possible. We show that for general probabilistic theories this heavily curtails the non-classical dynamics. We prove that there is a trade-off with the uncertainty principle that allows theories to evade this restriction. On one extreme, non-classical theories with maximal certainty have their non-classical dynamics absolutely restricted to only the identity operation. On the other extreme, quantum theory minimizes certainty in return for maximal non-classical dynamics.


Discord as a consumable resource

Pacific Rim Conference on Lasers and Electro-Optics, CLEO - Technical Digest (2013)

M Gu, HM Chrzanowski, SM Assad, T Symul, K Modi, TC Ralph, V Vedral, PK Lam

Quantum discord is conjectured to be a more general quantum resource than entanglement. We support this conjecture by showing, via experimental Gaussian optics, that quantum processors can harness discord to perform tasks classical counterparts cannot. © 2013 IEEE.


A framework for phase and interference in generalized probabilistic theories

New Journal of Physics 15 (2013)

AJP Garner, OCO Dahlsten, Y Nakata, M Murao, V Vedral

Phase plays a crucial role in many quantum effects including interference. Here we lay the foundations for the study of phase in probabilistic theories more generally. Phase is normally defined in terms of complex numbers that appear when representing quantum states as complex vectors. Here we give an operational definition whereby phase is instead defined in terms of measurement statistics. Our definition is phrased in terms of the operational framework known as generalized probabilistic theories or the convex framework. The definition makes it possible to ask whether other theories in this framework can also have phase. We apply our definition to investigate phase and interference in several example theories: classical probability theory, a version of Spekkens' toy model, quantum theory and box-world. We find that phase is ubiquitous; any non-classical theory can be said to have non-trivial phase dynamics. © IOP Publishing and Deutsche Physikalische Gesellschaft.


Local characterization of one-dimensional topologically ordered states

PHYSICAL REVIEW B 88 (2013) ARTN 125117

J Cui, L Amico, H Fan, M Gu, A Hamma, V Vedral


Experimental generation of quantum discord via noisy processes

Physical Review Letters 111 (2013)

BP Lanyon, P Jurcevic, C Hempel, M Gessner, V Vedral, R Blatt, CF Roos

Quantum systems in mixed states can be unentangled and yet still nonclassically correlated. These correlations can be quantified by the quantum discord and might provide a resource for quantum information processing tasks. By precisely controlling the interaction of two ionic qubits with their environment, we investigate the capability of noise to generate discord. Firstly, we show that noise acting on only one quantum system can generate discord between two. States generated in this way are restricted in terms of the rank of their correlation matrix. Secondly, we show that classically correlated noise processes are capable of generating a much broader range of discordant states with correlation matrices of any rank. Our results show that noise processes prevalent in many physical systems can automatically generate nonclassical correlations and highlight fundamental differences between discord and entanglement. © 2013 American Physical Society.


Extracting quantum work statistics by single qubit interferometry

Optics InfoBase Conference Papers (2013)

V Vedral

We first derive the Jarzynski relation [1] between the average exponential of the thermodynamical work and the exponential of the difference between the initial and final free energy. We then comment on the information-theoretic underpinning of Jarzynski's reasoning which helps explain why the Jarzynski relation holds identically both quantumly and classically [2]. We then present a scheme to verify the quantum non-equilibrium fluctuation relations as encapsulated by Jarzynski. We show that the characteristic function of the work distribution of a quantum system (which is basically equal to the Wick rotated average exponential of the thermodynamical work) can be extracted from Ramsey interferometry of a single probe qubit (which need not itself be pure, though it must not be fully depolarized) [3,4]. An interesting fact is that while the quantum version of the Jarzynski equality remains satisfied even in the presence of quantum correlations, the individual thermodynamical work moments in the expansion of the free energy are, in fact, sensitive to the genuine quantum correlations [5]. Whether this is a fortuitous coincidence remains to be seen, but it certainly goes towards explaining why the laws of thermodynamics happen to be so robust as to be independent of the underlying micro-physics. We intend to elucidate the subtle connection between Jarzynski's relation and the "quantum arrow of time". Finally we comment on the fact that our scheme for measuring the quantum work characteristic function belongs to the computational class known as DCQ1 [6], namely all computations that can be performed with only one pure qubit (and N maximally mixed ones). © OSA 2013.


Extracting quantum work statistics by single qubit interferometry

Optics InfoBase Conference Papers (2013)

V Vedral

We first derive the Jarzynski relation [1] between the average exponential of the thermodynamical work and the exponential of the difference between the initial and final free energy. We then comment on the information-theoretic underpinning of Jarzynski's reasoning which helps explain why the Jarzynski relation holds identically both quantumly and classically [2]. We then present a scheme to verify the quantum non-equilibrium fluctuation relations as encapsulated by Jarzynski. We show that the characteristic function of the work distribution of a quantum system (which is basically equal to the Wick rotated average exponential of the thermodynamical work) can be extracted from Ramsey interferometry of a single probe qubit (which need not itself be pure, though it must not be fully depolarized) [3,4]. An interesting fact is that while the quantum version of the Jarzynski equality remains satisfied even in the presence of quantum correlations, the individual thermodynamical work moments in the expansion of the free energy are, in fact, sensitive to the genuine quantum correlations [5]. Whether this is a fortuitous coincidence remains to be seen, but it certainly goes towards explaining why the laws of thermodynamics happen to be so robust as to be independent of the underlying micro-physics. We intend to elucidate the subtle connection between Jarzynski's relation and the "quantum arrow of time". Finally we comment on the fact that our scheme for measuring the quantum work characteristic function belongs to the computational class known as DCQ1 [6], namely all computations that can be performed with only one pure qubit (and N maximally mixed ones). © OSA 2013.


Comment on "quantum szilard engine"

Physical Review Letters 111 (2013)

M Plesch, O Dahlsten, J Goold, V Vedral

A Comment on the Letter by S. W. Kim, Phys. Rev. Lett. 106, 070401 (2011).PRLTAO0031-900710.1103/PhysRevLett.106.070401 The authors of the Letter offer a Reply. © 2013 American Physical Society.

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